Tides

Tides

I. Diurnal/semidiurnal/mixed patterns

Diurnal Tides: One high tide and one low tide per lunar day.
Characteristics:

  • Single high and single low tide in ~24 hours.

  • Typical of some Gulf of Mexico and Southeast Asia locations.

Cause: Gravitational and geographic factors that favor one tidal bulge.

Semidiurnal Tides: Two high tides and two low tides of approximately equal height per lunar day (~24 hours 50 minutes).
Characteristics:

  • High tide → low tide → high tide → low tide roughly every 12 hours.

  • Common along eastern coasts of North America and Western Europe.

Cause: Alignment of moon and sun producing two tidal bulges (one facing the moon, one opposite)

Mixed Tides: Two high tides and two low tides per lunar day, but heights of successive high tides and low tides differ

Characteristics:

  • Often occurs along Pacific coasts (e.g., California, western Canada)

  • One high tide is higher than the other (higher high tide vs. lower high tide)

  • One low tide is lower than the other (lower low tide vs. higher low tide)

Cause: Combination of semidiurnal tidal forces with local geography and bathymetry, causing asymmetry


II. Spring/Neap Tides

Spring Tides: Tides with the largest tidal range (highest high tides and lowest low tides)

Occurrence:

  • During full moon and new moon

  • The sun, moon, and Earth are aligned (syzygy)

Cause: Gravitational forces of moon and sun combine, reinforcing each other

Characteristics:

  • High high tides and very low low tides

  • Strong tidal currents


Neap Tides: Tides with the smallest tidal range (lowest high tides and highest low tides)

Occurrence:

  • During first quarter and third quarter moon

  • The sun and moon are at right angles relative to Earth

Cause: Gravitational forces of moon and sun partially cancel each other

  • Moderate tidal heights

  • Weaker tidal currents


III. Resonance

Tidal Resonance: Occurs when the natural period of oscillation of a body of water (e.g., a bay, estuary, or continental shelf) matches the period of the incoming tidal forcing.

  • Amplifies tidal ranges, producing higher-than-normal high tides and lower-than-normal low tides.

Mechanism

  1. Natural Oscillation

    • Each enclosed or semi-enclosed body of water has a resonant period determined by its length, depth, and shape.

  2. Tidal Forcing

    • The gravitational pull of the moon and sun produces tides with a specific period (~12.42 hours for semidiurnal tides).

  3. Resonance

    • If the water body’s natural period ≈ tidal period, wave amplitudes increase due to constructive interference.

Effects

  1. Higher tidal ranges in resonant basins.

  2. Enhanced tidal currents → stronger erosion, mixing, and sediment transport.

  3. Can influence navigation, coastal engineering, and flooding risks.

Examples

  1. Bay of Fundy (Canada) – tidal range ~16 m, amplified by resonance in the bay’s funnel shape.

  2. Severn Estuary (UK) – tidal range ~14 m, also resonant.

IV. Tidal Currents

The horizontal flow of water associated with rising and falling tides.

  • Unlike tidal heights (vertical motion), tidal currents represent movement of water parallel to the coastline.

Types of Tidal Currents

  1. Flood Current

    • Water moves toward the shore as the tide rises.

    • Strongest at mid-tide in many locations.

  2. Ebb Current

    • Water moves away from the shore as the tide falls.

    • Often stronger than flood in certain estuaries.

  3. Slack Water

    • Period when water stops moving between flood and ebb.

    • Occurs at high tide and low tide.

    • Short duration, can be used for safe navigation.

Characteristics

  1. Velocity: Varies with tidal range, basin shape, and friction.

  2. Reversal: Currents reverse direction twice a day in semidiurnal tides, once in diurnal tides.

  3. Influence of Geography: Constrictions, channels, and estuaries can accelerate tidal currents.

Effects

  1. Navigation

    • Strong tidal currents affect ship movement and port operations.

  2. Erosion & Sediment Transport

    • Move sand and silt → shape estuaries, tidal flats, and deltas.

  3. Ecology

    • Distribute nutrients and oxygen in intertidal zones and estuaries.

  4. Energy Potential

    • Tidal currents can be harnessed for tidal stream power.